1. Field of the Invention
The present invention relates to a heterojunction bipolar transistor (hereinafter, referred to as “HBT”) using a substrate formed with an epitaxial layer and a method for fabricating the same.
2. Description of Related Art
In order to realize a high-performance HBT, a process has been conventionally used in which a part of a collector region of a HBT located immediately below a base region thereof is inactivated by ion implantation to reduce the parasitic base-to-collector capacitance.
However, in the above-mentioned process, respective crystallinities of an emitter layer and a base layer and the crystallinity of the interface between the emitter layer and the base layer are damaged, resulting in deteriorated transistor characteristics (current amplification factor hFE=collector current/base current). The reason for this is that dopant ions implanted into the emitter surface passes through the base region and reaches the collector region.
On the other hand, in a method developed by Yamaguchi (Fujitsu Limited) (see Japanese Unexamined Patent Publication No. 6-232148), the formation of a sidewall made of an oxide film on the side of an emitter prevents the occurrence of the above-mentioned problem.